At least in regard to the Linux kernel, the interesting thing about this code base is that you can study the different sections separately. For instance the networking code, the VFS, the scheduler, are quite isolated from other stuff. However this also means that there is a variety of authors writing a variety of unrelated parts, in practical terms is a non uniform code base so it's hard to have an evaluation that works for everything. Btw, some of the parts I liked less are the ones written by Linus :-) Because he tends to make things a bit implicit in certain cases apparently. The VFS layer is an example.